This invention relates generally to the field of disc drive storage devices, and more particularly, but not by way of limitation, to improvements in the writing of servo data to discs of a disc drive.
Disc drives are commonly used in workstations, personal computers, laptops and other computer systems to store large amounts of data in a form that can be made readily available to a user. In general, a disc drive comprises one or more magnetic discs that are rotated by a spindle motor at a constant high speed. The surface of each disc is divided into a series of data tracks which are spaced radially from one another across a band having an inner diameter and an outer diameter. The data tracks extend circumferentially around the discs and store data in the form of magnetic flux transitions within the radial extent of the tracks on the disc surfaces.
Typically, each data track is divided into a number of data sectors that store fixed sized blocks of user data. Embedded among the sectors on each track are servo fields that enable the disc drive to control the position of heads used to transfer the user data between the discs and a host computer. More particularly, the heads are mounted to a rotary actuator assembly which includes a coil of a voice coil motor, so that the position of the heads relative to the tracks can be maintained by the application of current to the coil by a closed loop digital servo system in response to the servo information read by the servo fields. For a discussion of a typical digital servo system, see U.S. Pat. No. 5,262,907 issued Nov. 16, 1993 to Duffy et al., assigned to the assignee of the present invention.
The servo fields are written to the discs during the manufacture of the disc drives using a highly precise servo track writer, which utilizes the heads of the disc drive to write the servo fields. As the servo fields are used to define the tracks, it is important to precisely control the position of the heads as the servo fields are written to the disc surfaces. Thus, a typical servo track writer comprises a closed loop, laser based positioning system having a pusher pin which engages the actuator assembly and advances the position of the heads, a position detector which detects the position of the pusher pin and a motor which corrects the position of the pusher pin in response to correction signals from the position detector. Additionally, a typical servo track writer includes control circuitry for providing the servo information to be stored in the servo fields. For a more detailed discussion of typical servo track writing (STW) methodologies, see U.S. Pat. No. 5,164,863 issued Nov. 17, 1992 to Janz and U.S. Pat. No. 5,241,430 issued Aug. 31, 1993 to Janz, both of which are assigned to the assignee of the present invention.
As will be recognized, proper radial alignment of the servo fields is essential to facilitate reliable operation of the disc drive. If errors are introduced in the placement of the servo fields, components at corresponding frequencies can appear in a position error signal (PES) generated by the servo system during subsequent operation of the drive. The PES is a measure of the relative position of a selected head with respect to an associated track and is used primarily during track following operations to maintain the head over the center of the track. Such frequency components appearing in the PES for a selected track will result in the repeated adjustment of the position of the head by the servo system in an attempt to maintain the head over the center of the track during each revolution of the disc. When such frequencies are sufficiently severe, the correction required to account for these frequencies can require a significant amount of the total track misregistration budget, limiting the overall track density that can be achieved in a disc drive design.
One particularly pervasive source of errors induced during the STW process is the spindle motor, which includes bearing assemblies with characteristic frequencies that are generated from the rotation of the balls and ball cages within the inner and outer bearing raceways. These bearing frequencies are predictable and repeatable, and are not necessarily synchronized with the rotational frequency of the discs. These bearing frequencies can result in low frequency errors (artifacts) being laid down in the servo pattern. These artifacts result even if a separate motor is used to rotate the spindle motor during the STW process.
Accordingly, there is a need for an improved approach to reducing errors as servo data are written to discs of a disc drive to reduce or eliminate the effects of frequency components in a PES generated from the servo data during subsequent disc drive operation.
The present invention is directed to an apparatus and method for writing servo data to a disc of a disc drive.
In accordance with preferred embodiments, a disc pack comprising a disc and a spindle motor is mounted in a stationary disc servo track writing (STW) fixture, which maintains the disc in a nonrotating, stationary relation while rotating a servo write head around a circumference of the disc to write the servo data to the disc. Once the servo data are written and verified, the disc is removed from the STW fixture and installed into a disc drive.
The STW fixture preferably comprises a stationary base portion, top and bottom clamp members which extend from the base portion to hold the disc in the nonrotating, fixed relation, and a rotatable enclosure, adjacent the base portion, which supports the servo write head and rotates with respect to the base portion. The enclosure preferably uses air bearings to rotate with respect to the base portion.
By holding the disc in a stationary relation, the spindle motor bearings are not rotated, which eliminates the introduction of low frequency spindle bearing artifacts in the resulting servo data. Greater manufacturing efficiencies are also achieved since the servo data are written at the disc pack level instead of at the disc drive level as in the prior art, allowing improvements in rework, throughput and automation.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.